Audio microphones are commonly used in a variety of consumer applications such as cellular telephones, digital audio recorders, personal computers and teleconferencing systems. In particular, lower-cost electret condenser microphones (ECM) are used in mass produced cost sensitive applications. An ECM microphone typically includes a film of electret material that is mounted in a small package having a sound port and electrical output terminals. The electret material is adhered to a diaphragm or makes up the diaphragm itself.
Another type of microphone is a microelectro-mechanical Systems (MEMS) microphone, in which a pressure sensitive diaphragm is etched directly onto an integrated circuit. As such, the microphone is contained on a single integrated circuit rather than being fabricated from individual discrete parts.
Most ECM and MEMS microphones also include a preamplifier that can be interfaced to an audio front-end amplifier via a cord and plug for a target application such as a cell phone or a hearing aid. In many cases, the interface between the preamplifier and front-end amplifier is a three-wire interface coupled to a power terminal, signal terminal and ground terminal. In some systems, however, a two-wire interface is used in which the power and signal terminals are combined into a single wire, thereby reducing the cost of the system by using two wires instead of three wires.
Combining a power and signal interface into a single interface, however, poses a number of design challenges with respect to maintaining good audio performance in the presence of circuit noise, power supply noise and disturbances.